What Is a Confined Space Gas Detector?

The gas ionization detector refers to a radiation detector using a gas as a detection medium. Because the amount of charge collected is related to the electric field strength between the two electrodes, gas ionization detectors with different modes of operation are formed. The ionization chamber, proportional counter and GM (Geiger) counter tube are collectively called a gas ionization detector. Although the working characteristics of these three types of gas ionization detectors are not exactly the same, they all have one thing in common: the radiation ionizes the working gas in the detector, and then collects the generated electric charges to achieve the purpose of recording rays.

There are two types of ionization chamber: pulse ionization chamber and current ionization chamber. The pulse ionization chamber records the ionization effect caused by the incidence of a single particle in pulses, and is used to measure the relative activity and ray energy of the sample. This application has been largely replaced by semiconductor detectors. The current ionization chamber records the total average ionization current generated by a large number of particles entering the ionization chamber for a period of time, which reflects the total ionization effect and is used to measure the flux, dose, etc. of X, , rays or neutrons. A calibrated current ionization chamber can also measure radioactivity. Selecting an equivalent material of appropriate thickness as the electrode can measure the absorbed dose and exposure. Because it is simple and reliable, it is commonly used in monitoring applications. The pocket cumulative ionization chamber is also used as a personal dose pen. ^[1]

Proportional counter tube detector output pulse radiance is larger than the ionization chamber, and is proportional to the original ionization number. It can be used for both ray activity measurement and energy measurement. Resolution time is also short (

~

s) for quick counting. Suitable for detecting low-energy particles, X and rays. Commonly used in stationary or portable or personal beta, gamma contamination testing, garbage bag monitoring, nuclear site monitoring, etc. There are also 2 and 4 gas flow proportional counters for absolute activity and equal measurement. The positive electrode of some proportional counters is composed of multiple anode wires. It is called a multi-wire proportional counter. It has high detection efficiency, good spatial and temporal resolution, and is receiving increasing attention in nuclear physics, astrophysics, and biomedicine. Such as X-ray photography, positron photography, etc., can obtain organs, tumors and other clear images and analysis of protein structure. The disadvantage of the proportional counter is the strict requirements for the stability of the high-voltage power supply and linear amplifier. ^[1]

And the self-quenching streamer counting tube Geiger-Muller counting tube is referred to as Geiger counting tube, or GM counting tube. It was named after the inventor H. Geiger et al. Its structure is similar to a proportional counter tube, but with a stronger electric field around the anode wire. At this time, the ionization caused by the incident particles forms an avalanche phenomenon along the entire anode wire, and the amplitude of the output voltage pulse has nothing to do with the energy and properties of the incident particles. Generally it is around a few volts to a few volts. Therefore, it is possible to record with simpler electronic circuits. Another feature of the Geiger counter tube is that a small amount of quenching gas must be added to the tube or an external quenching circuit can be used to restore it to a normal state after a discharge, and a new incident time can be reached after a recovery time of about 100 microseconds Particles are counted, so they are limited in use.

It has been found that when the proportion of quench gas in the counting tube is large and under atmospheric pressure, the discharge method is essentially different from the Geiger discharge. It only produces local avalanches, so the recovery time is much shorter than the Geiger counter tube. This counting tube is called a self-quenching streamer counting tube. It can also give larger output pulses, and it is also independent of the initial ionization. Its pulse duration is only tens of nanoseconds. In addition, a self-quenching streamer can be created for positioning measurement. Due to the above-mentioned series of advantages, the self-quenching streamer has developed rapidly, and there is a tendency to replace the Geiger counting tube. ^[1]

Radiation detector with gas as detection medium. Its basic working principle is to ionize gas molecules when charged particles pass through the gas, and the number of ion pairs generated is related to the energy lost by the particles. If two electrodes are set in the gas ionization space and a certain potential difference is maintained, the electrons (or negative ions) and positive ions in the ion pair will be pulled apart by the electric field and drift in the direction of the electric field. The charge is collected by two electrodes and gives a certain electrical signal. Because the amount of charge collected is related to the electric field strength between the two electrodes, gas ionization detectors with different modes of operation are formed.

The gas detector is a cylindrical closed container filled with gas inside. The container has two electrodes that are insulated from each other. The metal cylinder is the cathode, and the metal wire in the cylinder is the anode. A DC high voltage is applied between the two electrodes. When no rays are incident into the gas ionization chamber, the gas is not ionized and there is no current in the circuit. After the switch is closed, under the action of the adjustable high voltage, a high voltage is formed on the anode, and an electric field is formed in the cylinder. When the ray enters the ionization chamber and the gas is ionized, under the effect of the electric field force, positive ions move toward the cathode, and negative ions or electrons move toward the anode. The collection of these charges causes the voltage across capacitor C to decrease, thereby obtaining an electrical pulse signal on the resistor that can be measured by an external circuit. When the adjustable voltage increases, the electric field becomes stronger. A sufficiently strong electric field can make the moving electrons or ions get enough energy to undergo further ionization (called secondary ionization), thereby increasing the number of ion pairs. ^[2]